1. Field of the Invention
This invention relates to a positive resist composition which is highly sensitive to high energy radiation such as deep-ultraviolet line, electron beam and X-ray, can be developed with alkaline aqueous solution to form a pattern, and is thus suitable for use in a fine patterning technique.
2. Prior Art
To comply with the LSI technology tending toward higher integration and higher speed, there were recently developed new resist materials which undergo acid-catalyzed chemical amplification. See Liu et al., J. Vac. Sci. Technol., Vol. B6, 379 (1988). Because of many advantages including a sensitivity at least comparable to conventional high-sensitivity resist materials, high resolution, and high dry etching resistance, the chemically amplified resist materials are regarded promising for deep-ultraviolet lithography. As to negative resists, a three-component chemically amplified resist material comprising a novolak resin, a melamine compound and a photo acid generator is commercially available from Shipley Co. under the trade name of SAL601ER7. At to positive resists, however, there have been commercially available no positive resist materials of the chemical amplification system. In the manufacture of LSIs, interconnection and gate formation can be managed using negative resists whereas the use of negative resists for contact hole formation obstructs fine processing due to fogging. There is a strong demand for high performance positive resist material.
In the past, Ito et al. developed a chemically amplified positive resist by adding an onium salt to a polyhydroxystyrene resin whose hydroxyl group is protected with a t-butoxycarbonyl (t-BOC) group, which is known as PBOCST. See Polymers in Electronics, ACS symposium Series No. 242, American Chemical Society, Washington D.C., 1984, page 11. Since the onium salt used therein contains antimony as a metal component, the PBOCST resist is not desirable from the standpoint of preventing contamination to the substrate.
Ueno et al. reported a deep-UV, chemically amplified positive resist based on poly(p-styreneoxytetrahydropyranyl) as having high sensitivity and high resolution. See No. 36 Applied Physics Society Related Joint Meeting, 1989, 1p-k-7. However, it was difficult to form a fine, high aspect ratio pattern at a high precision because of the mechanical strength thereof.
Many other reports have been made about such chemically amplified positive resist materials using novolak resins and polyhydroxystyrene as a base resin and being sensitive to deep-UV line, electron beam, and X-ray. All of them are single-layer resists. There still remain unsolved problems associated with substrate steps, standing wave resulting from optical reflection at the substrate, and difficulty to form high-aspect ratio patterns. These resist materials are unacceptable in practice.
The two-layer resist technique is recommended in order to form high-aspect ratio patterns on stepped substrates. To enable alkali development in the two-layer resist technique, silicone polymers having a hydrophilic group such as hydroxyl and carboxyl groups are required. Since the silicones having a hydroxyl group directly attached thereto, however, undergo crosslinking reaction in the presence of acid, it is difficult to apply such silanols to chemically amplified positive resist materials. While polyhydroxy-benzylsilsesquioxane is known as a stable alkali soluble silicone polymer, its derivatives obtained by protecting some hydroxyl groups with t-BOC form chemically amplified silicone system positive resist materials when combined with photo acid generators as disclosed in Japanese Patent Application Kokai (JP-A) No. 118651/1994 and SPIE, Vol. 19, 25 (1993), 377. However, silicone polymers are inadequate for fine patterning since they are less compatible with photo acid generators.
The above-mentioned resist materials also suffer from the problem of post-exposure delay (PED) inherently associated with chemically amplified positive resists. The PED problem is that after exposure, impurities such as amines and water in air diffuse into the resist film from its surface to create a distribution of impurities in the film. Because of the likelihood of acid being deactivated by such impurities at the surface, the pattern tends to have a "T-top" profile. There are available no silicone system positive resists which have solved the T-top problem associated with PED. There is a demand for a silicone system positive resist having high sensitivity and resolution.